As a power tool for use in fastening a screw, a bolt or the like, an impact tool has been known in which a rotation force by a motor is transmitted to a rotating hammer so that, by making the hammer to strike an anvil, the force is converted into an impact force. As such an impact tool, Patent Literature 1 proposes a tool in which cams that convert the rotation movements of the hammer to retreating movements in the axial direction through steel balls are respectively formed on a spindle and the hammer so that, when a predetermined fastening toque has been reached, the hammer retreats to release the meshed state between an anvil and a claw portion of the hammer, and by stored energy of the spring accumulated at the moment when the hammer retreats, the rotation energy of the hammer is energized to allow the hammer to strike the anvil, thereby fastening or loosening the bolt.